The knee joint is where the end of the upper leg bone (femur) meets the top of the lower leg bone (tibia). The end of the femur consists of two condyles, which are like runners or wheels on each side of the bone that both roll and slide with respect to the other surface. These condyles are located on top of the tibia, which is like a platform that is slightly dished on the medial side but slightly convex on the lateral side. Thus, on each side of the joint, there is an area of contact between the two bones. When the knee is bent, the condyles of the femur roll and slide on top of the tibia at these two areas of contact. A third bone, the kneecap (patella), glides over the front and end of the femur.
The two condyles form two articular bodies of the femur and are respectively named its lateral (outer) and medial (towards the middle) condyles. These diverge slightly distally (front) and posteriorly (rear), with the lateral condyle being wider in front than at the back while the medial condyle is of more constant width. The radius of the condyles' curvature in the sagittal plane becomes smaller toward the back. This diminishing radius produces a series of involute midpoints (i.e. located on a spiral). The resulting series of transverse axes (along the involute midpoints) permit the sliding and rolling motion in the flexing knee while ensuring the collateral ligaments are sufficiently lax to permit the rotation associated with the curvature of the medial condyle about a vertical axis.
Motion of a natural knee is kinematically complex. During a relatively broad range of flexion and extension, the articular or bearing surfaces of a natural knee experience rotation, medial and lateral angulation, translation in the sagittal plane, rollback and sliding. The knee joint, in combination with ligaments and muscles, attempt to produce this complex knee motion, as well as absorb and control forces generated during the range of flexion.
In a healthy knee joint, the surfaces of these bones are very smooth and covered with a tough protective tissue called cartilage. Arthritis causes damage to one or more of the bone surfaces and cartilage where the bones rub together. In particular, osteoarthritis or “wear-and-tear” arthritis is a condition where the surface of the joint is damaged and the surrounding bone grows thicker, as a result of bone against bone friction. This friction can cause severe pain and eventually loss of movement.
One option for treating the permanent deterioration of the cartilage is total knee replacement surgery. In a total knee replacement procedure the end surfaces of the tibia and femur, and the posterior surface of the patella, are resurfaced. Generally, the surface that covers the femoral section is made from smooth metal or ceramic, while the surface attached to the tibia is constructed of metal and a high-density polyethylene. The patella is also resurfaced with a high-density polyethylene.
While known knee joint prostheses have proven to be effective in replacing the anatomical knee joint, they nevertheless have several disadvantages. For example, knee joint prostheses sometimes lack adaptability to implant conveniently with a given patient. In this regard, in a normally shaped femur, the central canal is typically offset from the centre of the femoral articulating surfaces. Furthermore, the central femoral canal may present various valgus angles from one patient to another.
In some cases, there may be significant damage on only one side of the joint or to only one of the condyles. In these cases, a partial (unicompartmental) knee replacement may be considered. In a partial knee replacement, only one side of the joint is resurfaced.
Accordingly, it is desirable to develop a method for knee surgery, and in particular, a method of implanting a unicondylar knee prosthesis which may help make surgery, recovery, and rehabilitation faster and easier for the patient and that seeks to preferably mitigate, alleviate or eliminate one or more of the disadvantages mentioned above singly or in any combination.
Any discussion of documents, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art.